1. Field of the Invention
The present invention is directed to a process of preparing a ferritic alloy formed in its surface with an oxide scale of alumina having improved wear resistance as well as mechanical strength.
2. Description of the Prior Art
Ceramics or alloys have been utilized in the art as forming the cutting tools and the mechanical elements such as gears and shafts requiring increased wear resistance. The ceramics is known to give an increased wear resistance or surface hardness as high as 2000 Hv, but suffers from the inherent disadvantage of insufficient toughness resulting in crack or fracture. On the other hand, the alloys are known to have sufficient toughness but to give only poor surface hardness as less as 1100 Hv. Consequently, it has been proposed to provide on the alloy surface an extra layer of superior wear resistance, for example, TiN or ZrN layer by spattering or chemical vapor deposition (CVD) techniques. Such extra layer is, however, found to have rather less adherence to the base alloy, in addition to that it is difficult to be formed into sufficient thickness, which result practically in poor performance.
To overcome the above disadvantages, the recent technology has proposed hot oxidation resistive alloys which forms in its surface an aluminum oxide scale under hot oxidation atmospheres. For example, the preceding U.S. application Ser. No. 604,231, now U.S. Pat. No. 5,089,223, discloses ferritic alloys capable of forming in its surface an aluminum oxide scale, i.e., alumina scale with improved wear resistance, toughness, and superior scale adherence to the matrix. The ferritic alloy is prepared by casting an alloy liquid composition in a mold to obtain an ingot which is subsequently machined into a desired shape. Then, the resulting product is heated in an oxidizing atmosphere to form in the surface thereof the alumina scale. With this process, it is difficult not only to give a complicate shape to the product but also to obtain an increased thickness of the alumina scale due to the fact that the oxidization is substantially limited to the skin-deep area in the surface of the alloy product. In other words, because of the dense surface structure of the cast product, the oxidizing gas is only supplied to the surface of the product such that the oxidization into the alumina scale will be soon saturated in the skin-deep area to thereby inhibit further formation of the alumina scale in the deep area. Further, since the thickness of the alumina scale is substantially dependent only upon the hot corrosion or oxidization condition, there is less flexibility in control the thickness of the aluminum oxide scale.